1. Field of the Invention
This invention relates to a bobbin case adjustment mechanism, referred to in the sewing machine industry as a cushion spring assembly, for use in a sewing machine having a vertical axis rotary hook.
2. Description of the Prior Art
In a sewing machine that has a vertical axis hook, the bobbin case must be restrained from rotation within the hook while still leaving enough space between the bobbin case and the rotation restraining mechanism to allow a loop of needle thread to pass between the restraining mechanism and the hook during the process of forming stitches.
Some prior bobbin case rotation restraining mechanisms were fastened to the sewing machine bed by screws or similar fasteners that precluded adjusting their positions to correct for errors due to construction and assembly tolerances incurred during the manufacture of the restraining mechanism.
Another prior bobbin case rotation restraining mechanism shown in Japanese Utility Model Application No. 52-74864 incorporated, at one extremity thereof, a stud with an eccentric portion that permitted the mechanism to be pivotally moved toward or away from the bobbin case to effect limited control over the width of a slot located between the bobbin case rotation restraining member and the bobbin case and through which the needle thread must pass during its concatenation with the bobbin thread. While the means for pivotal adjustment incorporated in that mechanism made the machine less prone to thread jamming than rotation restraining mechamisms that did not incorporate adjustment means, the adjustable means can only be adjusted along a limited line, or arc, but cannot be adjusted to all of the points around a closed loop. Such freedom of movement is necessary to allow the cushion spring to be moved in two dimensions relative to the position of the bobbin case to permit accurate setting of the dimensions of more than one part of the slot.